1. Field of the Invention
The invention relates to a permanent magnet machine with an asymmetric rotor design.
2. Background Art
A permanent magnet electric machine typically has a rotor comprising a ferrous metal core and embedded sets of permanent magnets. A stator, which surrounds the rotor, is comprised of ferrous metal and windings with multiple-phase alternating current. Such machines, which sometimes are referred to as interior permanent magnet machines, are capable of functioning as a motor or as a generator. The sets of magnets are inserted in holes or cavities in the rotor core.
The weight of the rotor and the moment of inertia of the rotor can be reduced by providing channels in the rotor core. The channels are also used for accommodating cooling oil flow for heat dissipation for some applications. These channels or “oil holes” typically are located symmetrically adjacent rotor magnetic poles for the permanent magnets. The permanent magnet machine will have reduced effective electromagnetic performance for a given stator current if the size of the holes causes a significant reduction in the width of a magnetic flux flow path in a flux-flow region of the rotor. The location of the holes also affects the flux-flow path.
If the permanent magnet machine is used in a hybrid electric vehicle powertrain, weight reduction, thermal management and current capacity of the electric permanent magnet machine are of particular importance and should be taken into account in arbitrating the effects of changes in a flux-flow distribution on overall performance.
In a known permanent magnet machine used in a hybrid electric vehicle powertrain, round oil holes are used, and an electromagnetic flux-flow path is created around the oil holes. The oil holes are positioned symmetrically with respect to embedded permanent magnets.
If oil holes are not included in the rotor design and the permanent magnet machine does not require them for cooling or weight reduction purposes, the magnetic flux in the region below the permanent magnets has a wide path through which the flux passes. Thus, the flux saturation level in that region will be low. Further, the saturation level will be relatively homogenous due to a large cross-sectional area that is available for the flux flow path. When oil holes are introduced in the rotor design, however, the magnetic flux is forced to flow through a path that is much narrower at locations between the magnets and the oil holes. A resulting increase in saturation level that is created requires higher currents to compensate for an electromagnetic torque reduction.